Haze is caused by the scattering and absorption of visibility light by particles and gases. Light is an electromagnetic wave, and just as a plane water wave is deflected by a barrier from its original direction, light waves are scattered when they encounter particles and gas molecules that are approximately the same size as the light's wavelength. The sky is blue because particle-free air also scatters light, but the gas molecules are so small that they scatter the shorter wavelength blue light more than they scatter the longer wavelength red light. Total extinction (b_(ext)), expressed in inverse megameters (Mm~(-1)), is multiplied by the distance between an observer and a target to indicate the amount of light removed from the site path by scattering and absorption by particles and gases, and is the most common measurement used to quantify visibility. Total extinction does not take into consideration light scattered into the sight path that further reduces the target's contrast (ratio of light transmitted from the target to the light transmitted from the horizon). Defining natural conditions is a scientific challenge. Annual average estimates are currently in use, but these will eventually need to be made more event-specific. Wildfires, dust storms, and other natural events will affect visibility on a case-by-case basis. Transport from outside of provincial and national boundaries needs to be considered because this is largely beyond the control of authorities.
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